Polyoxoarsenotungstate in Oxalis Triangularis Arrangement Favors Slow Magnetic Relaxation for Both Prolate and Oblate Lanthanides

A series of Ln(III)-substituted polyoxometalates (POMs) having molecular formula CsxK15−x[(AsW9O33)3Ln2(H2O)7W4O9]·yH2O were synthesized by using dilacunary arsenotungstate [As2W19O67(H2O)]14− as a precursor with Ln(III) ions [Ln = Dy (1), Tb (2), Er (3), Ho (4), Yb (5), Eu (6), x = 6−7, y = 24−40]. The Ln-POM complexes comprise a fusion of three trivacant arsenotungstates (ATs) [α-AsW9O33]9− building units resembling an oxalis triangularis-like arrangement sealing a heterometallic [Ln2(H2O)7(W4O9)]12+ core unit. Detailed magnetic studies disclose that complex 1 shows slow relaxation of magnetization at zero and 2000 Oe external dc fields due to its high axial anisotropy. An energy barrier of 56.48 K is obtained under the 2000 Oe applied dc field, whereas only the QTM process is favorable in the absence of dc field. On the other hand, complex 5 shows field-induced slow relaxation of magnetization with 5 and 6 K phenomenological energy barriers under 1500 and 2000 Oe applied dc fields. Furthermore, ab initio calculations confirm that high axial ground states get stabilized with minimum transverse anisotropy for complex 1 whereas easy plane anisotropy is observed for complex 5. It is also observed that the arrangement of the POM cluster provides intermediate axial symmetry (C2v) around the metal centers, stabilizing higher mj states along with high anisotropy for both Dy(III) and Yb(III) ions in complexes 1 and 5. Thus, both Dy(III) (oblate ion) and Yb(III) (prolate ion) Ln-POM clusters display slow magnetic relaxation, despite having different spatial distributions of 4f electronic density. The solid-state photoluminescence spectra of complexes 1−4 and 6 were investigated, which displays the characteristic emission of Ln(III) components based on 4f−4f transitions.